on someone's sideidiom
Definition of on someone's side
1 : supporting someone in his or her opinion, position, etc. Are you on my side or his? Whose side are you on, anyway?
2 : as an advantage He is not a very tall basketball player, but he does have quickness on his side.
3 : helping someone to succeed Luck seems to be on your side tonight. Time is on our side. We may well finish the project on time.